Copolymers of ethylene with alpha, beta-unsaturated acids and unsaturated esters are well-known in the art and a large number of such copolymers are available commercially, for example ethylene/methyl acrylate/methacrylic acid and ethylene/vinyl acetate/acrylic acid copolymers. In the past one of the problems associated with continuous processes for preparation of such compositions in stirred tank reactors was the tendency for large amounts of polymeric deposits to form on the inner walls of the reactors. This buildup, which consists of highly crosslinked, insoluble polymer, i.e. gel, adversely affected operation of the reactor. In addition, particles of the gel were invariably introduced into the product which tended to compromise the quality of the copolymer produced. It has recently been found that this problem can be alleviated by performing the copolymerization in the presence of small amounts of methanol or acetone, as disclosed in U.S. Pat. Nos. 5,028,674 and 5,057,593. Due to the chain transfer activity of methanol and acetone, however, in some cases the polymeric products thus obtained are of extremely low viscosity, so low in fact that they are unsuitable for many applications.
A highly effective process for increasing viscosity of such copolymers is disclosed in co-pending U.S. patent application Ser. No. 07/769,603 whereby a free radical generating agent is added to the copolymer in an amount sufficient to decrease its melt index to 0.1-15 g/10 minutes while producing a copolymer product having a gel content of less than 3%, as determined by measurement of % insolubles in boiling xylene. The process thus provides a method for increasing viscosity without concomitantly raising the gel content of the polymer or producing products which contain domains of highly crosslinked gel within the polymer matrix.
Under certain circumstances however, free radical generating agents can have deleterious effects on the physical properties of the product, for example decrease in tensile and tear strengths. In order to mitigate these effects it would therefore be desirable to employ a method to increase raw polymer viscosity which minimizes the amount of free radical generating agent used.